The present invention relates generally to gas turbine engines, and, more specifically, to reduction of exhaust noise and infrared (IR) signature.
A turbofan aircraft gas turbine engine includes in serial flow communication a fan, compressor, combustor, high pressure turbine, and low pressure turbine. Air is pressurized in the compressor and mixed with fuel and ignited in the combustor for generating hot combustion gases that flow downstream through the turbine stages which extract energy therefrom for powering the compressor and fan.
The combustion gases are exhausted from the turbines through a downstream core engine exhaust nozzle. And, fan air bypasses the core engine and is discharged through an annular fan exhaust nozzle surrounding the core engine. In this way, two streams of fan exhaust and core exhaust are discharged coaxially from the engine at corresponding velocities which create noise.
Government regulations limit acceptable levels of engine noise as aircraft take off and land at airports, Accordingly, aircraft engines are specifically designed for limiting the amount of noise generated therefrom during operation.
Noise attenuation may be effected by mixing the high velocity core exhaust with the surrounding lower velocity fan exhaust for reducing the peak velocity thereof, and correspondingly reducing noise as well as IR signature. And, the fan exhaust may be mixed with the surrounding lower velocity ambient air for reducing the peak velocity thereof and the associated noise.
In U.S. Pat. No. 6,360,528, a chevron exhaust nozzle is disclosed in various configurations for the core nozzle and fan nozzle for attenuating noise. Triangular chevrons extend aft from an annular exhaust nozzle, and have compound curvature both axially and circumferentially for enhancing mixing of the two flow streams located radially inwardly and outwardly thereof.
The present invention is a continuation in the development of the chevron exhaust nozzle in which a particular problem was discovered and is solved under the present invention. The chevron nozzle has been built and tested in annular form and provides substantial reduction in noise and IR signature.
However, in some mounting configurations of a turbofan aircraft engine with the chevron exhaust nozzle, not only was noise not reduced, but actually increased due to the introduction of the chevron nozzle.
More specifically, a turbofan engine may be mounted to an aircraft using a pylon under a wing or along the side of the fuselage. The pylon is a structural member with a fairing skin which extends along the longitudinal axis of the engine at one circumferential location such as about twelve o=clock for a wing mounted engine, and either three o=clock or nine o=clock for fuselage mounted engines. The pylon thus locally blocks a portion of the fan nozzle and may extend aft from the core nozzle in various lengths and widths.
For example, in one relatively wide and long pylon extending over the core nozzle, testing of an annular chevron nozzle therefor resulted in an increase of noise instead of the intended decrease of noise. Similarly, testing of a chevron fan nozzle interrupted by the pylon failed to produce expected noise attenuation.
Accordingly, it is desired to provide an improved chevron exhaust nozzle for use in conjunction with pylon mounted aircraft engines.
An exhaust nozzle includes an annular exhaust duct for discharging exhaust from an aft end thereof. Circumferentially adjoining chevrons extend from the duct aft end around only an arcuate portion thereof leaving a plain arcuate shelf between terminal ones of the chevrons. Each of the chevrons has a triangular configuration and a compound arcuate contour both circumferentially and axially.